The present invention relates to an air-fuel ratio control system for an internal combustion engine having an emission control system with a three-way catalytic converter for controlling air-fuel ratio correcting means in accordance with conditions of engine operation by detecting cooling water temperature and load on the engine.
Conventionally, this type of air-fuel ratio control system achieves a feedback control by detecting the air-fuel ratio from oxygen concentration in exhaust gases by an O.sub.2 sensor. The detected air-fuel ratio is determined by a comparison circuit as to whether it is lean or rich comparing with the stoichiometric air-fuel ratio to produce an output signal. The output signal is sent to an electro-magnetic valve through a driver to actuate the valve for supplying a certain amount of air to a carburetor, and controlling the air-fuel ratio of the air-fuel mixture to the stoichiometric air-fuel ratio. The air-fuel ratio control system of such construction functions in good condition when the engine operates at a proper engine temperature. However, under cold engine operating condition, it is difficult to control the air-fuel ratio in accordance with the engine operation. Therefore, in an engine having an automatic choke device, the air-fuel ratio control system is so arranged that the system is rendered inoperative in the cold engine, while the air-fuel ratio of the mixture is controlled by the operation of the automatic choke device. However, the automatic choke device tends to open the choke valve before a proper time or to open it wider than a degree proper to the warming-up of the engine. Due to such difficulties, in the conventional air-fuel ratio control systems, the air-fuel ratio is always controlled to the stoichiometric air-fuel ratio even when the engine is in cold operating condition. Therefore, although during the cold engine operating condition at a constant speed with a light load, it produces an output torque sufficient to give a steady driveability, during half-open throttle or wide-open throttle operation, stumble operation of the engine occurs so that the output of the engine decreases.
This is because, despite the condition that the engine requires a rich air-fuel ratio during cold engine, mixture of air-fuel ratio leaner than that of the engine requirement is supplied to the engine by the air-fuel ratio control system, and further the air-fuel ratio is shifted to lean side when the automatic choke valve is forced opn by the drag of the air which is sucked in during acceleration.